Asphaltic concrete pavement and method of preparing same



United States Patent 3,201,269 ASPHALTEC CONCRETE PAVEMENT AND METHGD 0FPREPARENG SAME Philip C. Doyle, Rocky River, Ohio, assignor to TheStandard Gil Company, Cleveland, Ghio, a corporation of Ohio N0 Drawing.Filed Mar. 12, W53, Ser. No. 264,762 1 Claim. (Cl. 106-28tl) Thisapplication is a continuation-in-part of application Serial No. 26,970,filed May 5, 1960, and now abandoned.

This invention relates to an improved asphaltic cement for pavingpurposes. More particularly, this invention relates to a method ofimproving an asphalt cement by retarding the loss of ductilecharacteristics which normally occurs during high temperature mixingwith aggregate so as to improve the service behavior of an asphalticpavement in which said asphalt cement is incorporated.

Good ductility is recognized by those skilled in the art as an importantand desirable property for anasphalt cement intended for pavingpurposes. It is believed that the ductility of an asphalt cement isrelated to the actual service behavior of an asphaltic cement in whichit is incorporated so that as a qualitative matter the better theductile characteristics of the asphalt cement present in the asphalticpavement, the better Will be the service behavior of the pavement.

In my paper entitled Cracking Characteristics of Asphalt Cement, whichappears in volume 27 of the Pro ceedings of the Association of AsphaltPaving Technologists, at page 581, I propose that the ductilecharacteristics of asphalt cement are meaningful with respect to thetendency of an asphaltic pavement in which it is incorporated tofracture or crack in service, particularly if the ductilecharacteristics of the asphalt are measured on a ductilometer at lowertemperatures and speeds than those commonly employed. In this papertests are reported on core samples which were drilled from pavementsactually in service, showing both good and bad resistance to surfacecracking. It was found from these various tests that when the ductilecharacteristics of the asphalt cement recovered from these core sampleswere run at 55 F., and 1 cm./min..speeds on ductilometer the ductilityvalues of the asphalt cement could be closely correlated with theservice behavior of the pavement. By comparison, it was found that whenthe ductility of the recovered asphalt was determined at temperatures of77 F. and at speeds of 5 cm./min. the ductility values obtained were notalways consistent with field results with respect to the crackingtendency of the pavement. Besides showing that ductility measurements ofthe asphalt cement at 55 F. and 1 cm./ min. speeds on the ductilometeroffer a convenient means for predicting the service behavior of anasphaltic pavement, the results reported in my paper for these lowtemperatnure ductility measurements stress the importance of preservinggood ductile properties in the asphalt in order to insure that theasphaltic pavement will provide good service behavior.

It has been found that asphalt cements generally have initially goodductile characteristics so that if this high degree of ductility werepossessed by the asphalt after being laid into a pavement, good servicebehavior of the pavement could be expected. However, it is well knownthat the ductility of the asphalt cement is materially lost during thehigh temperature mixing of the asphalt cement with aggregate so that theductile characteristics of the asphalt cement in the pavement is usuallyof a relatively low value. For example, a typical asphalt for pavinghaving a penetration value of 7085 measured at 77 F. obtained from aMississippi-Arkansas-Illinois crude had a ductility of 150+ cms. whenmeasured on a ductilometer at 55 F., 1 cm. speed. This asphalt, however,after Patented Aug. 17, 1965 ICC being mixed with gravel aggregate in astandard mix mill at 310 F. in accordancewith conventional tradepractices showed a ductility value of only 10 cm. when measured at 5 F.and 1 cm. speed.

This, of course, demonstrates the drastic eflfect the high temperaturemixing has on the ductile characteristics of an asphalt cement. It isbelieved that the loss in ductile characteristics which occurs duringthe mixing operation is due primarily to an oxidation mechanism inducedby the high temperatures that must be employed. It is to be understood,however, that the present invention is not at all dependent upon whatmechanism is responsible for this loss in ductile value since the factremains that such a loss does occur. There is, of course, someprogressive hardening and loss of ductility in the laying of the asphaltand in its exposure to the elements while present in the pavement, butthis loss is negligible in relation to the loss which occurs during themixing operation. Consequently, if some way could be found to inhibitthe loss occurring during the mixing operation, the ductilecharacterisitcs of the asphalt could be largely preserved and thepavement in which it is incorporated would have greater resistance tocracking.

Generally, the degree of ductility loss in the mixing operationincreases with the increase of temperature and/ or time employed inmixing and the time the finished mixture is held in a truck at themixing temperature prior to its actual laying. According to good tradepractices, the temperature at which a mix mill is operated is generallyselected as the temperature which will give seconds viscosity on aSaybolt-Furol viscometer for the particular asphalt that is used. It isbelieved that this temperature provides the best film thickness ofasphalt on the aggregate for paving purposes. However, since few mixmills are equipped with thermocouples, the operating temperature inactual practice of these mills has a tendency to vary over wide limitsand the problem of the loss of ductile characteristics in the asphaltcement becomes particularly aggravated. Also, since the locations ofthese hot mix paving plants are frequently distant from the paving job,it often happens during cold climatic conditions that the asphalt mustbe mixed at much higher temperatures than the proper mixing temperaturein order that the paving mix will arrive at the job at a sufiicienttemperature for paving. Moreover, frequently the time of mixing is notcarefully or uniformly controlled at the mix mill with commensuratedepreciation of the ductile temperatures or at proper mixingtemperatures but for excessive mixing periods.

The primary object of the present invention, therefore, is tominimizethe loss of ductile characteristics in an asphalt cement whichoccurs during the high temperature mixing with aggregate so as toimprove the asphalts resistance to cracking in the pavement. It is afurther purpose of this invention to permit much greater latitude in thetime and temperature of mixing of asphalt with aggregate. A stillfurther object of this invention is to permit asphalt cements which areoriginally borderline with respect to ductile characteristics to be usedin paving operations where they formerly could not be employed.

These objects and other objects which. will become apparent from thefollowing discussion may be accomspouses 6 plished in accordance withthe present invention by adding to the asphalt cement before the mixingoperation a phosphorus compound selected from the following generalformula:

wherein R is a phenyl radical or an alkyl substituted phenyl radicalcontaining up to a total of 12 carbon atoms in the alkyl substituent orsubstituents, and preferably not more than 4. R may be, for example,phenyl, tolyl, xylyl, butyl phenyl, hexyl phenyl, 2-ethyl hexyl phenyl,dodecyl phenyl, butyl octyl phenyl, or butyl hexyl phenyl. Thephosphorus compounds of the invntion are well known to the art and arecommercially available. Such materials are readily prepared by reactingphenols or alkylphenols with phosphoric acid according to known methodsor by any other suitable means forming no part of the present invention.

In order to gain any significant effect, at least 0.01% by Weight basedon the weight of the asphalt cement of one or a mixture of phosphatecompounds from the foregoing general formula must be used. Actually,there is no upper limit but amounts over 2% by weight usually cannot bejustified economically. A preferred range is generally from 0.5% to 1%by weight.

The asphalt ingredient of this invention may be any natural ormanufactured bituminous material which may be mixed with any of thecommon aggregates, such as crushed limestone, slag, crushed rock, sand,gravel, etc. to form an asphaltic concrete for paving. The desirableproperties for such an asphalt cement may be found in highwayspecification manuals or in Abrahams text Asphalt and Allied Substances.In general, asphalt cement forpaving purposes is required to meetpenetration specifications and the preferred penetration will usually befrom 50-200 at 77 F.

, In preparing an asphaltic concrete for paving, the phosphate compoundis introduced into the asphalt cement at some convenient point beforethe cement is charged to the mix mill, employing any suitable mixingmeans which will insure that the additive is evenly distributedthroughout the asphalt cement. The cement is then mixed with aggregatein the proportions of from 4 to 8 parts by weight of asphalt cement to96 to 92 parts by weight of aggregate, with heating at an elevatedtemperature in the range of from 280 F. to 350 F. As indicatedhereinabove, the preferred temperature vithin this range is the onewhich gives a viscosity of 120 seconds on the Saybolt- Furol viscometer,for the particular asphalt cement used.

The preparation of an asphaltic concrete pavement in cludes theadditional steps of transporting the mixture, While still at an elevatedtemperature, from the site of mixing to the'place where the mixture isto be laid as a pavement, and laying the mixture as an asphalticconcrete pavement. In order to allow for proper handling the temperatureof the mixture at the place it is to be laid as a pavement should not bebelow 250 F.

For best results, the pavement should be built up from one or morelayers of asphaltic concrete each of which has been compacted to athickness of 1 to 3 inches, and contains not more than about voids. Thephosphate additive functions during the mixing and transporting of theasphaltic concrete at an elevated temperature to inhibit a ductilityloss in the asphalt in the finished pavement.

A better understanding of the present invention Will be gained from thefollowing discussion of Oven Weather Tests comparing the loss of ductilecharacteristics during mixing with aggregate of an untreated asphaltcement with the same asphalt treated with a phosphorus corn; pound ofthe invention.

The Oven Weather Test simulates the high temperature mixing in aconventional mill and correlates therewith. Results from this test haveindicated that ductile characteristics measured on a sample of asphaltcement recovered from the hot paving mix prepared and heated inaccordance with this test at 55 F, 1 cm. speed, parallel the ductilecharacteristics When measured at 55 F, 1 cm. speed, for a sample of thesame asphalt cement recovered from a hot paving mix just before it islaid as a pavement following mixing in a conventional mix mill at theproper temperature to give 120 seconds viscosity (Saybolt-Furol) for theasphalt cement employed and including an average of approximately onehour transportation of the hot mixture to the job site.

In executing this oven test, 3000 grams of standard Ottawa sand is mixedwith 120 grams of asphalt cement for two minutes at the temperaturewhich gives 120 seconds viscosity as determined by means of aSaybolt-Furol viscometer for the asphalt cement of the test. Immediatelyafter this mixing, the mixture is spread to a uniform depth in shallowaluminum pans measuring approximately 5 /2" in diameter and /8" in depthand allowed to cool to room temperature for at least one hour. Theasphaltic mixture is scraped from the pans and the asphalt is recoveredby extraction with benzene in accordance with Absons Method and Testwhich is recorded in detail beginning at page 48 of The Association ofAsphalt Paving Technologists Proceedings for 1952, vol. 21. The originalasphalt and the recovered asphalt, on evaporation of the benzene, weretested for ductility according to ASTM designation Dll3-44 at 55i0.9 F.and at a rate of speed of 1 cm./min.

In the table below, the oven weathering of an asphalt cement having apenetration of -100 at 77 F., obtained from aMississippi-Arkansas-lllinois crude is compared with the same asphaltafter treating with tri-tolyl phosphate and tri-phenyl phosphate. Ineach case the phosphorus compound was added to the asphalt cement Whileit was maintained in a molten and mobile condition. A mechanical stirrerwas provided to insure that the phosphorus compound was evenly dispersedthroughout the asphalt.

it is obvious from these data that the phosphorus compounds of theinvention markedly reduce the loss in ductile characteristics duringhigh temperature mixing with aggregate.

it is to be understood that various modifications of the foregoinginvention will occur to those skilled in the art upon a reading of thedescription. All such modifications are intended to be included as maybe reasonably covered by the appended claim.

I claim:

A method for preparing an asphaltic concrete pavement exhibitingimproved retained ductility comprising the steps of adding to an asphaltcement having a penetration of from 50-200 at 77 F, in an amount withinthe range of 0.0l to 2 wt. percent based on the Weight of asphaltcement, a phosphorus compound of the general formula:

6 wherein R is selected from the group consisting of phenyl, ReferencesCited by the Examiner and alkyl substituted phenyls in which the alkylsub- UNITED STATES PATENTS stituent contains a total of up to 12 carbonatoms, mixing from 4 to 8 parts by Weight of said asphalt cement con-2693425 11/54 Hardman 106 '273 talning said phosphorus compound Withfrom 96 to 92 5 OTHER R CE Parts y Weight of aggregate at an elevatedtempefatllfe Page 528 and chapter XXVIII, volume one, fifth ediof from280 F. to 350 F., and While still at said eletion, Abraham, Asphalts andAllied Substances, New vated temperature, transporting the mixture fromthe site York, D. Van Nostrand Company, Inc.

of mixing to the place the mixture is to be laid as an asphalticconcrete pavement, laying the mixture as an 10 ALEXANDER BRODMERKELPrimary Examine"- asphaltic concrete pavement. MORRIS LIEBMAN, Examiner.

